Harmonic class c amplifier



May 10, 1949.

DRIVING SOURCE A. S. HARRIS HARMONIC CLASS C AMPLIFIER Filed April 25, 1947 FIG.| 22

VOLTAGE OF PLATE TANK 4 VOLTAGE OF LOAD TANK 27 GRID VOLTAGE \PLATE CURRENT TIME INVENTOR.

ALBERT S. HARRIS ATTORNEY Patented May 10, 1949 HARMONIC CLASS C AMPLIFIER Albert S. Harris, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application April 25, 1947, Serial No. 744,003

12 Claims.

This invention relates to amplifier circuits and particularly to a class C amplifier having a high efiiciency.

It has long been recognized in the art that it is desirable to arrange a class C amplifier in such a manner that a small amount of heat only is developed at the plate of the amplifier tube. The amount of energy which is transformed into heat at the plate of the tube, and which must be dissipated, depends substantially upon the product of the instantaneous plate current and the instantaneous plate voltage with respect to that of the cathode. This may be easily understood because the product of the instantaneous plate current and the instantaneous plate voltage represents the kinetic energy of all the electrons which impact the plate of the tube, and this kinetic energy is substantially transformed into heat.

It will accordingly be obvious that the plate losses of a class C amplifier may be materially reduced by causing plate current to flow only during a short portion of the cycle, that is, during that portion when the plate Voltage is at a minimum. It has therefore long been recognized in the art that a class C amplifier should be driven in such a manner that plate current flows only during a very short portion of the cycle. This may, for example, be effected by impressing rectangular pulses on the control grid of a class C; amplifier, the pulses having a duration which is short compared to a cycle of operation. However, the drawback of this arrangement is that a large portion of the energy delivered to the plate circuit of the tube appears in the form of harmonic energy while the useful energy should appear at the fundamental frequency to which the plate circuit is tuned. This harmonic energy is due to the distortion of the plate voltage as can be shown by a Fourier analysis and is lost unless it can be stored in the condenser of the plate tank circuit and subsequently released at the fundamental frequency. The harmonic currents which flow in the inductive branch of the plate tank circuit are effectively coupled to the load, which is undesirable.

In order to decrease the plate losses it has been proposed to provide a number of tank circuits ar ranged in series in the plate circuit of the tube, one of the tank circiuts being tuned to the fundamental frequency while the other tank circuits are tuned to harmonics of the fundamental frequency. In a class C amplifier circuit of this type the harmonic tank circuits allow the plate voltage to drop to a very low value thereby to minimize plate losses and to reduce the generation of heat which must be dissipated by the plate of the tube. On the other hand, a considerable portion of the output power appears in the form of harmonic energy which cannot be utilized in the load impedance. Thus, the losses do not occur in the plate of the tube but in the tank circuits which are tuned to harmonies of the fundamental frequency. In a highly efficient class C amplifier the tank circuit coupled to the plate of the tube should have its highest impedance at the time the tube conducts space current. Furthermore, the energy which may be present in the form of harmonics of the fundamental frequency should not be impressed on the load impedance but should be transformed into useful energy at the fundamental frequency.

It is an object of the present invention, therefore, to provide a novel high efficiency class C amplifier circuit.

A further object of the invention is to provide a class C amplifier circuit having a tank circuit arranged so that its impedance is at a maximum when space current flows in the tube and where substantially all energy is impressed on the load at the fundamental frequency.

In accordance with the present invention there is provided, in a high efliciency amplifier circuit, a first and a second tank circuit. Means are provided for impressing pulses at a predetermined frequency on the first tank circuit for exciting it. Switching means such as a unilaterally conducting device are provided for conductively connecting the two tank circiuts and for delivering power from the first to the second tank circuit. The conductive connection of the two tank circuits by the switching means causes the two circuits to be disconnected When the pulses are impressed on the first tank circuit. Accordingly, the two tank circuits are only connected between the occurrence of the pulses for delivering power at the fundamental frequency of the pulses from the first into the second tank circuit.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. x "In the accompanying drawing:

Fig. 1 is a circuit diagram of a single ended class C amplifier circuit embodying the present invention;

Fig. 2 is a graph showing curves referred to in explaining the operation of the circuit of Fig. 1; and

Fig. 3 is a circuit diagram of a class C' amplifier circuit in accordance with the invention including two amplifier tubes connected in push-pull.

Referring now to Fig. 1, there is illustrated a single ended class C amplifier circuit in accordance with the invention including amplifier tube I having cathode 2, control grid 3 and anode 4. Cathode 2 is connected to ground, and a suitable source of grid bias voltage such, for example, as battery 5 is provided between cathode 2 and control grid 3. Control grid 3 is connected to the negative terminal of battery 5 through grid leak resistor 3. cathode 2, that is ground, and control grid 3 through coupling condenser 8. Driving source I may be arranged to develop a sinusoidal in put wave for driving amplifier tube I in the manner of a class C amplifier.

In order to increase the efficiency, driving source 'I .may be arranged to develop pulses such as indicated at Ill at a predetermined fundamental frequency. The width of pulses I deter-,

mines the conduction period of amplifier tube 1. It has already been pointed out that the efficiency of a class C amplifier increases when its conduction period decreases in view of the resulting reduction of the plate losses. On the other hand, when the amplifier is driven by pulses of small width compared to a cycle of operation, the output voltage will contain energy at harmonic frequencies.

Anode 4 is connected to a suitable source of anode voltage indicated at 'B+ through anode tank circuit Ii which includes inductance element I2 and condenser I3 arranged in parallel. Anode tank circuit 1 I has a high impedance and is tuned substantially to the fundamental frequency of the input wave which may be a sinusoidal wave or may consist of recurring pulses.

'In accordance with the present invention there is provided load tank circuit I5 consisting of "induc'tance element IB and condenser i! arranged in parallel. Load tank circuit I5 is preferably tuned to the fundamental frequency of the input wave. A load impedance indicated schematically by resistor I8 may be connected across the terminals of load tan'k circuit I'5 through blocking condenser 20. Tank circuits II and I5 are not inductively coupled to each other.

.A unilaterally-conducting device such as diode 2 I is provided for conductively connecting anode tank circuit II to load tank circuit I5. Diode 2-I has a cathode 22 connected to load tank circuit I5 and an anode 23 connected to anode tank circuit I I. Ihe other terminals of .both tank circuits I I and I5 are connected to the anode voltage supply B+ as shown. Accordingly, one terminal .of the two tank circuits H and I5 is maintained at a fixed potential, that is, at the potential of the anode voltage supply.

The operation of the class C amplifier circuit illustrated in .E'ig. v1 may be explained by reference to Fig. .2. Solid curve 25 in Fig. 2 indicates the voltage of anode '4 of amplifier tu'be I plotted against time. The voltage of anode 4 varies with time substantially like a sinusoidal wave. The input wave impressed on control grid 3 of amplifier tube I may consist of recurring pulses It! as illustrated in Fig. 2. Whenever the grid voltage exceeds the grid shutoff voltage, space current fiows through "amplifier I as indicated at 2.6 in Fig. 2. When control grid 3 is driven by pulses, the plate current also has the A driving source I is coupled between to load tank circuit I5.

shape of recurrent square topped pulses illustrated at 26.

As soon as the plate current begins to fiow, the plate voltage curve 25 is depressed as indicated at 2? corresponding to the additional charge which flows into anode tank circuit II. The voltage of load tank circuit I5 is indicated by dotted curve 28 which, as shown, is more nearly sinusoidal than plate voltage 25. An inspection of Fig. 2 shows that the voltage across load tank circuit I5 is positive with respect to that across anode tank circuit II during the negative portion of the cycle, and accordingly, current can no longer fiow through diode 2I. Thus, tank circuits II and I5 are effectively disconnected during substantially the entire negative portion of the cycle. Therefore, anode tank circuit II has its highest impedance during the time space current flows through amplifier tube I. When tank circuits I I and I5 are disconnected, harmonic energy is prevented from being coupled to load impedance I8, that is, to load tank circuit I5. Substantially the entire energy represented by the plate current is accordingly stored in anode tank circuit II during the time diode 2I is not conducting.

After the occurrence of plate current pulse 2'6, the voltage across anode tank circuit II rises again rapidly and will eventually correspond with the rising voltage of load tank circuit I5, that is, curves 25 and 28 will merge. At this point the two tank circuits II and I5 are again conductlvely connected through diode 2| and power is delivered from anode tank circuit II The energy delivered from anode tank circuit II to load tank circuit 15 is substantially at the fundamental frequency as evidenced by the sinusoidal shape of the voltage wave curve 28 of load tank circuit I5. Most of the energy delivered by the plate current at harmonic frequencies into anode tank circuit II is stored in condenser I3 of the anode tank circuit and released again at the fundamental frequency.

It will accordingly be seen that the efficiency of the amplifier circuit of Fig. 1 is substantially increased because anode tank circuit II has its highest impedance during the time that space current flows through amplifier tube I. Furthermore, a large portion of the energy flowing into anode tank circuit II at harmonic frequencies is released again and transferred to load tank circuit I5 at the fundamental frequency.

Preferably plate tank circuit II is tuned to a frequency which is slightly higher than that of the input wave while load tank circuit I5 should be tuned to the fundamental frequency of the input wave. When anode tank circuit I I is tuned to a frequency that is above that of the input wave, the voltage across the anode tank circuit will already have reached or approached the minimum at the time the-plate current begins to fiow which should decrease the anode losses. It :is to be understood, however, that anode tank circuit II may alsobe tuned to the fundamental frequency of the input wave as is conventional.

It is to be understood that the amplifier circuit :of'Fig. 1 .may be modulated in any .conventional manner by either modulating the grid voltage or the plate voltage of amplifier tube I.

-Referring now to Fig. 3., there is illustrated a class C amplifier circuit in accordance with the present invention comprising two amplifier tubes 38 and 31 arranged in push-pull. The cathodes of amplifier tubes 30 and 3! are con- 5 nected to ground, and a suitable grid bias voltage is impressed between the cathodes and control grids of the two amplifier tubes by battery 32 having its negative terminal connected to the control grids by grid leak resistors 33 and 3 4, respectively. An input wave which may consist of pulses shown. at 35 and 3b is impressed on the control grids of amplifier tubes 3|! and 3| in such a manner that the two tubes conduct space current alternately. Alternatively, a sinusoidal input wave may be impressed in phase opposition on the control grids of amplifier tubes 36 and 3| in a conventional manner.

Anode tank circuit 31 is connected across the anodes of amplifier tubes 3% and 3| and comprises inductance element 38 and condensers 48, 4| having their junction point grounded. The cen ter tap of inductance element 38 is connected to the anode voltage supply B-lthrough choke 42 which, however, may be omitted.

Load tank circuit 43 comprises inductance element 44 and condenser d5 arranged in parallel. The center tap of inductance element M is connected to 3+ through choke 46 which also may be omitted, if desired. The load impedance indicated by resistor 41 may be coupled across load tank circuit 43 by two blocking condensers it, 43. Two diodes 5|! and 5| interconnect the terminals of the two tank circuits 3'? and 63 for the same purpose as explained in connection with Fig. l.

The circuit of Fig. 3 operates as follows. Assume that amplifier 31) is just conducting space current. Accordingly, diode 5| will not conduct current because the voltage at the junction point of diode 5| and load tank circuit 43 will be more positive than that between the junction point of diode 5| and anode tank circuit 37!. A the same time, however, diode 59 will conduct space current because at this time the junction point between load tank circuit 43 and diode 553 will be negative with respect to that of the junction point between diode E and anode tank circuit 31.

When amplifier 3|) ceases to conduct space current while amplifier tube 3| conducts space current, the conditions are reversed so that diode 58 will not conduct current while diode iii is conducting. It will be understood that the circuit of Fig. 3 is not as efficient as that of Fig.

1 because diodes 51! and conduct alternately so that a portion of the harmonic energy developed across anode tank circuit 31 will be im pressed upon load tank circuit 43 which is not desirable. However, since condensers ll) and 4| of anode tank circuit 37! have their junction point grounded, a large portion of the harmonic energy will be stored in condensers iil and M and released later at the fundamental frequency. In other words, the harmonic currents flow into either condenser 4d or 4| depending upon which condenser at that moment is connected to the tube drawing current.

Diode 2| as well as diodes 5d and 5! should have low losses so that the voltage between the plate and the cathode of the diodes may be relatively low when they conduct current. If diodes 2|, 5!] and 5| are of the indirectly heated type, their heater windings must be floating and connected to their respective cathodes.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modificatlons may be made therein without departing from the invention, and it is, therefore, aimed 6. in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a high efiiciency amplifier circuit, a first and a second tank circuit, means for impressing pulses at a predetermined frequency on said first tank circuit for exciting it, and switching means automatically operable at the frequency of said pulses for conductively connecting said two tank circuits and for delivering power from said first to said second tank circuit.

2. In a high efiiciency amplifier circuit, a first and a second tank circuit, said first tank circuit having a high impedance, means for impressing pulses at a predetermined frequency on said first tank circuit for exciting it, a load impedance coupled to said second tank circuit, and switching means operable in response to the voltages of said tank circuits for normally conductively connecting said two tank circuits and for disconnecting them substantially during the time said pulses are impressed on said first tank circuit, thereby to deliver power from said first to said second tank circuit.

3. In a high efficiency amplifier circuit, a first and a second tank circuit, said first tank circuit having a high impedance, means for impressing pulses at a predetermined frequency on said first tank circuit for exciting it, a load impedance coupled to said second tank circuit, said tank circuits being tuned substantially to the frequency of said pulses, and a unilaterally conducting device for conductively connecting said two tank circuits and for deliverin power from said first to said second tank circuit.

4. In a high efiiciency amplifier circuit, a first and a second tank circuit, said first tank circuit having a high impedance, means for impressing pulses at a predetermined frequency on said first tank circuit for exciting it, a load impedance coupled to said second tank circuit, said first tank circuit being tuned to a frequency which is higher than that of said pulses, said second tank circuit being tuned to the frequency of said pulses, and

a unilaterally conducting device for conductively connecting said two tank circuits and for delivering power from said first to said second tank circuit.

5. A high efiiciency amplifier circuit comprising a class C amplifier tube having a control element, a cathode and an anode, means for impressing an input wave between said control element and said cathode, a source of anode voltage coupled to Said anode, a first and a second tank circuit, each being tuned substantially to the frequency of said input wave, said first tank circuit being coupled to said anode, and switching means for normally conductively connectin said two tank circuits and for disconnecting them substantially during the time when said amplifier tube is conducting current, thereby to deliver power from said first to said second tank circuit.

6. A high efiiciency amplifier circuit comprising a class C amplifier tube having a control element, a cathode and an anode, means for impressing an input wave between said control element and said cathode, a source of anode voltage coupled to said anode, a first and a second tank circuit, each being tuned substantially to the frequency of said input wave, said first tank circuit being coupled to said anode, and a unilaterally conducting device for conductively connecting said two tank circuits and for delivering power from said first to said second tank circuit.

7. A high efiiciency amplifier circuit comprising a class C amplifier tube having a control element, a cathode and an anode, means for impressing an input wave between said control element and said cathode, a source of anode voltage conductively connected to said anode, a first and a second tank circuit, said first tank circuit being tuned to a frequency which is higher than that of said input wave, said second tank circuit being tuned to the frequency of said input wave, said first tank circuit having a high impedance and being coupled to said anode, and a unilaterally conducting device for conductively connecting said two tank circuits and for delivering power from said first to said second tank circuit. 7

8. A high efiiciency amplifier circuit comprising a class C amplifier tube having a control element, a cathode and an anode, means for impressing an input wave between said control element and said cathode, a source of anode voltage, a first and a second tank circuit, each being tuned substantially to the frequency of said input wave, a load impedance coupled to said second tank circuit, said first tank circuit being connected between said source and said anode, and a diode having a cathode and an anode for conductively connecting said two tank circuits, the anode of said diode being connected to said first tank circuit, said second tank circuit being connected between said source and the cathode of said diode.

9. A high efiiciency amplifier circuit comprising two class C amplifier tubes connected in pushpull, each having a control element, a cathode and an anode, means for impressing an input wave between said control elements and said cathodes to drive said tubes alternately, a source of anode potential conductively connected to said anodes, a first and a second tank circuit, said first tank circuit being coupled to said anodes, and unilaterally conducting devices for conductively connecting said tank circuits and for delivering power from said first to said second tank circuit. I

10. A high eificiency amplifier circuit comprising two class C amplifier tubes connected in pushpull, each having a control element, a cathode and an anode, means for impressing an input wave between said control elements and said cathodes to drive said tubes alternately, a source of anode potential conductively connected to said 8 cuits and for delivering power from said first to said second tank circuit.

11. A high efiioiency amplifier circuit comprising two class C amplifier tubes, each having a cathode, a control element andan anode, said cathodes being connected together, means for impressing an input wave between said cathodes and said control elements for alternately driving said tubes, a source of anode potential, a first and a second tank circuit, said first tank circuit having a high impedance, means for connecting said source to said tank circuits, said first tank circuit being connected between said anodes, a load impedance coupled to said second tank cir cult, and two unilaterally conducting devices for conductively connecting said tank circuits and for delivering power from said first to said second tank circuit, said first tank circuit being tuned to a frequency which is higher than the frequency at which one of said tubes is conducting space current, said second tank circuit being tuned to the frequency of the current conduction of one of said tubes.

12. A high efficiency amplifier circuit compris ing two class C amplifier tubes, each having a cathode, a control element and an anode, said cathodes being connected together, means for impressing an input wave between said cathodes and said control elements for alternately driving said tubes, a source of anode potential, a first and a second tank circuit, each including an inductance element, said first tank circuit having a high impedance, said source being connected to the midpoint of each of said inductance elements, said first tank circuit being connected between said anodes, a load impedance coupled to said second tank circuit, and two diodes, each having a cathode and an anode, the cathodes of said diodes being connected between the terminals of the inductance element of said second tank circuit and the anodes of said diodes being connected between the terminals of the inductance element of said first tank circuit for conductively connecting said tank circuits and for delivering power from said first to said second tank circuit,

said tank circuits being tuned substantially to the frequency at which one of said tubes is conducting space current.

ALBERT S. HARRIS.

REFERENCES CITED UNITED STATES PATENTS Name Date Lyons Nov. 30, 1937 Number 

